The Rookery Building is a historic office building at 209 South LaSalle Street in the Loop community area of downtown Chicago, Illinois, United States. Completed by architects Daniel Burnham and John Wellborn Root of Burnham and Root in 1888, it is considered one of their masterpiece buildings, and was once the location of their offices. The building is 181 feet (55 m) in height, twelve stories tall, and is considered the oldest standing high-rise in Chicago. It features exterior load-bearing walls and an interior steel frame. Inside is a light court, which illuminates a two-story atrium lobby with ornamental stairs. The building is designated a Chicago Landmark, and it is listed on the National Register of Historic Places as a National Historic Landmark.

Edward C. Waller agreed to lease the site in 1885 for 99 years and hired Burnham and Root to design the building. The lobby was remodeled between 1905 and 1907 by Frank Lloyd Wright, and Wright assistant William Drummond remodeled the structure in 1931. The building was sold in 1982 to Continental Illinois, which cleaned the facade and resold it in 1989 to Baldwin Development. From 1989 to 1992, the lobby was restored to Wright's design. The building was resold twice in the 2000s; following the latter sale, the building was renovated yet again in the 2010s.

The Rookery is located at 209 South LaSalle Street in the Loop community area of downtown Chicago, Illinois, United States. It was built by the architectural partnership of Daniel H. Burnham and John Wellborn Root, known as Burnham and Root. The building, designed in the Chicago School architectural tradition, is one of the few results of their partnership that is still standing. While much of the Loop's 19th-century architecture has been demolished, the Rookery has been preserved and renovated over the years.

In the architectural boom that followed the Great Chicago Fire, architects in what would become the Chicago School competed with each other to create the world's first true skyscrapers. In their design for the building, Burnham and Root mixed modern building techniques, such as metal framing, fireproofing, elevators and plate glass, with traditional ones, such as brick facades and elaborate ornamentation. At the same time, they intended their buildings to be commercially successful. As the master artisan, Root drew upon a variety of influences in designing the interior and exterior spaces, including Moorish, Byzantine, Venetian and Romanesque motifs.

The Rookery Building rises 12 stories. The marble, granite, terracotta, and brick facade of the building is a combination of Roman Revival and Queen Anne styles that embraced Richardsonian Romanesque architecture. The facade is largely tinted in a reddish-brown hue. The lower two stories are clad in rough-faced granite, with columns made of red granite, while the upper stories are clad with brick. Thick blocks are used at the lower stories to give the facade the impression of sturdiness. Birds are carved into the facade near the entrance, an allusion to the building's name.

The structure uses a "floating" foundation—a reinforced concrete slab that provided the building's weight with a solid platform atop Chicago's notoriously swampy soil. Root designed a grillage foundation, in which iron rails and the structural beams are combined in a crisscross pattern and encased in concrete to support the building's immense weight without heavy foundation stones. This construction is particularly useful when structural loads are high compared to the natural bearing capacity of the soil.

The building uses a combination of cast and wrought iron framing and masonry bearing walls. This marked a transitional moment in a switch from masonry to steel skeleton structures, and, according to the architectural critic Paul Goldberger, was a precursor to the steel frames of early skyscrapers. The Landmarks Commission citation commends "development of the skeleton structural frame using cast iron columns, wrought iron spandrel beams, and steel beams to support party walls and interior floors". Metal-framed perimeter walls are used only on two sides of the building, and only on two floors; the rest of the walls are made of masonry. This was because the developers had been concerned that the metal frames were not sturdy enough to hold up the building's weight.

Root and Burnham designed a two-story central light court to serve as the focal point for the entire building and provide daylight to interior offices. The light court, clad in white terracotta, provides natural illumination for the interior offices. The skylight atop the light court is composed of 5,000 glass panes. The lower stories also had a retail arcade, described in one source as the first indoor arcade in the U.S..